FIG. 1 illustrates a telephone service arrangement between a subscriber 10 (e.g., a residential or commercial telephone customer) and a service provider 12 that exchanges data with a telephone company central office (TCCO) 14 to provide telephone service to the subscriber 10. There are many telecommunication standards that the service provider 12 should comply with to insure compatibility between telecommunication devices at the subscriber 10 and the service provider 12.
One of the standards with which the service provider 12 should comply is the Telcordia Standard TR-NWT-000057 (referred to herein as the “Telcordia Standard”), which specifies the impedance level a telecommunication device at the subscriber 10 should encounter when a connection is established with the service provider 12. According to the Telcordia Standard, this impedance level is 900 Ω+2.16 μF as viewed by the subscriber 10 between the tip line 16 and ring line 18 (referred to herein as the tip/ring lines 20). Telecommunication devices for use at the subscriber 10 are designed based on the impedance level set forth in the Telcordia Standard and, therefore, if the impedance of the tip/ring lines 20 deviates from this standard, telephone service may be affected adversely.
In traditional audio only telephone service arrangements (i.e., plain old telephone service, “POTS”), a subscriber line interface card (SLIC) 22 and a coder/decoder (CODEC) 24 generate a suitable impedance level between the tip/ring lines 20. The CODEC 24 develops a signal based on an output from the SLIC 22 at port VTX that reflects current sensed by the SLIC 22 at protected tip (PT) port and protected ring (PR) port. The developed signal can be fed back to the tip/ring lines 20 via the SLIC ports PT and PR to synthesize an impedance that complies with the Telcordia Standard, i.e., 900 Ω+2.16 μF. In a typical arrangement, the SLIC 22 receives the signal from the CODEC 24 through a non-inverting receive AC signal input (RCVP) and an inverting receive AC signal input (RCVN).
Recently, asynchronous digital subscriber line (ADSL) has become a common standard for transferring data at a very high rate between the subscriber 10 and the TCCO 14. ADSL service is provided over the same tip/ring lines 20 as POTS. The ADSL signals are transmitted in a frequency band above about 25 kHz, whereas traditional POTS signals are transmitted in a frequency band below about 4 kHz.
FIG. 2 illustrates an interface within a service provider 12 (FIG. 1) for separating ADSL and POTS signals received from the subscriber 10 for transmission to the TCCO 14, and combining ADSL and POTS signals received from the TCCO 14 for transmission to the subscriber 10. The interface circuit of FIG. 2 adds a transformer 26, which contains a transformer blocking capacitor 28, to the service arrangement of FIG. 1. Ideally, the transformer 26 exhibits a low impedance to signals in the ADSL band. The transformer blocking capacitor 28 is selected to prevent low frequency signals (e.g., signals in the POTS band) from passing through the transformer 26, thereby creating a “pure” ADSL signal for processing by ADSL circuitry 30 at the service provider 12 (FIG. 1). In addition, a low pass filter (LPF) 32, which contains a coupled inductor 34 and a capacitor 36, is added to filter out signals in the ADSL band, thereby creating a “pure” POTS signal for processing by the SLIC 22 and CODEC 24. A first resistor 38 is coupled between the PT port of the SLIC 22 and the LPF 32 and a second resistor 40 is coupled between the PR port of the SLIC 22 and the LPF 32 to provide protection for the SLIC 22. Also, a first protection circuit 42 and a second protection circuit 44 are coupled between the SLIC 22 and the tip/ring lines 20 to protect the SLIC 22 from voltage spikes created by the coupled inductor 34 of the LPF 32.
A problem that arises when the transformer 26 containing the transformer blocking capacitor 28 is inserted into the traditional POTS circuitry is that, at higher frequencies of the POTS band, e.g., above about 2 kHz, the transformer blocking capacitor 28 begins to pass AC current. Because current begins to flow through the transformer blocking capacitor 28 at these frequencies, the impedance of the tip/ring lines 20 is essentially the impedance developed by the CODEC 24 and SLIC 22 in parallel with the impedance of the transformer blocking capacitor 28. (The impedance through the windings of the transformer 26 is essentially zero at these frequencies.) This reduces the impedance of the tip/ring lines 20 at these higher POTS band frequencies, thereby adversely affecting the quality of the POTS.
Accordingly, methods and apparatuses are needed to compensate for the transformer blocking capacitor's effect on impedance for signals having frequencies in the POTS band, while not affecting the impedance for signals having frequencies in the ADSL band.